1. Field of the Invention
This invention relates to a method of refining steel.
2. Description of the Prior Art
The current practice in refining steel, either in an open hearth furnace or a basic oxygen converter, consists of additions of burnt lime, dolomitic lime, and fluorspar to form a basic slag that reacts with the oxidation products of silicon, carbon, manganese, phosphorus, sulfur, and incidental impurities from the charged materials which normally consist of steel scrap and hot metal from the blast furnace. Normally, the cold scrap is charged into the furnace or converter, the hot metal is then added, and the refining procedure commences. Burnt lime, dolomitic lime, and spar additions are made to the bath in order to provide the desired slag composition. The silicon in the hot metal is first oxidized and molten silica is formed and mixes with the mixture of burnt lime, dolomitic lime and fluorspar.
Inasmuch as burnt lime and dolomitic lime have melting points in excess of 4,500.degree. F, the particles or chunks of those slag-forming limes are substantially immersed in the molten silica which forms a layer of dicalcium silicate that completely envelopes the exterior of each basic particle of burnt lime and dolomitic lime. The resulting material is highly refractory and has an even lower rate of solubility than the original burnt lime and therefore inhibits the solution of lime and extends the time required to accomplish lime solution. This is perhaps the most significant barrier to the early creation of a desirable slag composition in the overall refining process.
When the slag conditioner and fluorospar are added to the conventional charge, the ratio of lime-to-silica is approximately one-ton-one. During the first stage of oxidation (silicon blow) silicon oxidizes to silica and lowers the ratio of lime to silica below the one-to-one proportion. As the amount of silicon available for oxidation diminishes and the amount of silica forming diminishes, and a minimum ratio, for example, of from 0.4 to 1, is reached, the basicity of the slag subsequently formed slowly increases as the solid basic particles of burnt lime and dolomitic lime begin to go into solution. Fluorspar has been used to speed up lime solution rate, but it does not prevent refractory encapsulation of the lime particles. It reacts to disrupt these envelopes and therefore its impact is confined to the latter stages of the process. Fluorspar particles have a high melting point and are also subject to encapsulation by the high silica liquid formed in the early stages of refining. Encapsulation impedes the rate of solution of lime particles and in like manner impedes the rate of solution of fluorspar. Therefore, steel producers are actively seeking a better material that will equal or exceed fluorspar's acceleration of lime solution rate and that will not have its harmful side effects. Further, fluorspar supply is limited and the price has increased continuously over the past 20 years. Currently from about 3 to 25 pounds of fluorspar are used per ton of steel produced depending on local steel works raw material parameters. There exits therefore a need for the introduction of a slag conditioner as a partial but substantial substitute for fluorspar.